what is vpn?

"Virtual" is a concept that is slightly more complicated.  The New Hacker's Dictionary (formerly known as the Jargon File) [2] defines

virtual as –

virtual /adj./ [via the technical term "virtual memory", prob.  from the term "virtual image" in optics] 1.  Common alternative to

{logical}; often used to refer to the artificial objects (like addressable virtual memory larger than physical memory) simulated by

a computer system as a convenient way to manage access to shared resources.   2.  Simulated; performing the functions of

something that isn't really there.  An imaginative child's doll may be a virtual playmate.  Oppose {real}.

Insofar as VPN's are concerned, the definition in  2.  above is perhaps the most appropriate comparison for virtual networks.  The

"virtualization" aspect is one that is similar to what we briefly described above as "private," however, the scenario is slightly modified – the

private communication is now conducted across a network infrastructure that is shared by more than a single organization.  Thus, the

private resource is actually constructed by using the foundation of a logical partitioning of some underlying common shared resource,

rather than by using a foundation of discrete and dedicated physical circuits and communications services.  Accordingly, the "private"

network has no corresponding "private" physical communications system.  Instead, the "private" network is a virtual creation which has

no physical counterpart.  The virtual communications between two (or more) devices is due to the fact that the devices which are not

participating in the virtual communications are not privy to the content of the data, and that they are also altogether unaware of the

private relationship between the virtual peers.  The shared network infrastructure could, for example, be the global Internet and the

number of organizations or other users not participating in the virtual network may literally number into the thousands, hundreds of

thousands, or millions.